In a variable-speed, constant-frequency (VSCF) power generating system, variable-frequency power produced by a brushless, synchronous generator is converted by a power converter into constant-frequency AC power. The power converter includes a rectifier bridge which converts the variable-frequency power into DC power on DC link conductors and an inverter which converts the DC power into the constant-frequency AC power. The inverter may be, for example, of the neutral point clamped type wherein positive and negative switch assemblies are connected in series across the DC link conductors and wherein a neutral switch assembly is connected between a neutral voltage and a junction between the positive and negative switch assemblies.
In general, each of the positive and negative switch assemblies includes driver and driven transistors connected together in a Darlington configuration together with associated base biasing, snubber and flyback components connected thereto. The neutral switch assembly includes all of these components with the exception of a flyback diode across the driven transistors. Also, the neutral switch assembly requires four high power diodes connected in a bridge configuration which permit bi-directional current flow between the neutral voltage and the phase output. The neutral switch assembly thus requires three additional power diodes as compared with each of the positive and negative switch assemblies.
The components of each switch assembly handle large magnitudes of current and are subject to failure. Such a failure may cause harmonic content in the inverter output to increase beyond acceptable limits or may render the entire inverter inoperative. In such a case, it is necessary to shut down the inverter in order to isolate and replace the failed component. The process of isolating the failed component can be lengthy, in turn leading to significant down time for the inverter.
The process of isolating a failed component can be shortened considerably by packaging each of the switch assemblies as a separate unit, such as on a single printed circuit board. Thus, for each inverter leg, only three components, (i.e. each switch assembly) need be tested in order to isolate a failed component. Once the failure is isolated to a single board, the entire board can be replaced and the faulty board repaired "off-line". While this approach minimizes the time that an inverter is out of service, it requires that at least two different sets of boards be used, one for the positive and negative switch assemblies and one for the neutral switch assembly. This, in turn, increases the number of boards which must be stocked for replacement purposes.